Thermosetting polymer composites offer a variety of advantages over metal structures including lighter weight and resistance to corrosion. Despite these advantages, polymer composites are combustible and this leads to concerns when these materials are used in enclosed environments such as aircraft, surface ships, or submarines. Many high temperature polymers are intrinsically fire resistant. In particular, polymers that contain primarily aromatic groups show excellent fire resistance and high char yields. Incorporating heteroatoms is another way to further increase the fire resistance of high temperature polymers. Most fire resistant additives are based on elements including boron, aluminum, phosphorus, antimony, chlorine, and bromine. Halogenated additives are the most prevalent and act via formation of gas phase radicals that scavenge hydrogen radicals and result in formation of non-flammable HCl or HBr that further dilute flammable oxidants. Although halogenated polymers or additives are effective at reducing flammability, the generation of toxic and corrosive byproducts reduces the attractiveness of these materials. In contrast, phosphorus based flame retardants act through formation of a surface glass that can protect the substrate from oxygen and flame, promote charring, and inhibit free radical propagation. These materials can also act through a vapor phase mechanism in which PO, P, and P2 species react with H and OH radicals to form HPO.
Cyanate ester resins are a well-known class of thermosetting polymers with high intrinsic fire resistance. A number of different approaches have been utilized to decrease the flammability of cyanate esters. One method involves the use of bisphenols that incorporate more rigid aromatic structures. For example, a cyanate ester with a 4,4′-biphenylene structure has been prepared with a char yield of 64% and a UL-94 rating approaching V-0. Another approach is to increase the crosslink density by increasing the number of crosslinking sites per molecule. For example, recently reported tris(cyanate) esters derived from resveratrol resulted in char yields >70% and a heat of combustion of only 2.5 kJ/g. Taking advantage of halogen-based materials, a cyanate ester derived from bisphenol C has been extensively studied. Other researchers have decreased the flammability of cyanate esters by incorporating nitrogen heterocycles. For example, Emrick synthesized a novel triazole containing cyanate ester that was non-flammable and had a heat release capacity of only 10 J/(g·K).
Combining a high temperature thermosetting resin with phosphorus-based flame retardants is another common route to the preparation of fire resistant composite materials. Polyphosphates and diphosphates prepared from bisphenol A have been blended with epoxy resins. Cyanate ester resins containing cyclic phosphinates that can be blended with conventional cyanate esters to generate V-0 grade composites have been prepared. Phosphinated cyanate esters have also been prepared from phenylphosphine oxides, while self-curing phosphine adducts of BADCy and blends with epoxy resins were studied by Lin. Although many of these materials have applications as fire-resistant materials that can be used as standalone resins or blended with conventional materials, there are currently no known examples of cyanate esters with a simple phosphate core.
Cyanate esters with enhanced flame retardancy have applications for electronic and aerospace applications. Several studies have shown that the flame retardancy of cyanate esters can be greatly enhanced through the incorporation of phosphorus. To date this has mostly been accomplished through a blending approach, but there are a few examples of cyanate ester resins that contain phosphate groups covalently bound to the aromatic rings. The current invention describes a simple method for synthesizing bis and tris-cyanate esters that have phosphate groups bridging between the aromatic rings. Through this approach, composite materials with exceptional resistance to fire can be fabricated.
It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not to be viewed as being restrictive of the invention, as claimed. Further advantages of this invention will be apparent after a review of the following detailed description of the disclosed embodiments, which are illustrated schematically in the accompanying drawings.